This invention relates generally to blades for cutting and perforating plastic films and more particularly to a blade for perforating thermoplastic films in the manufacturing of plastic bags.
In the manufacture of bottom sealed thermoplastic bags, it is necessary to process the bag through an apparatus which produces a heat seal and an adjacent perforated line across the tubular film. The heat seal provides the bottom closure for one bag and the perforations permit the adjacent bag to be separated by tearing along the perforations. The perforated web must have sufficient strength in the remaining tabs to maintain integrity until it is desired to separate the bags.
One common technique for heat sealing and perforating the bags is to index the tubular film as it emerges from the blown film extrusion system through an in-line bag machine, which is provided with a reciprocating serrated blade and sealing element. With a portion of the flattened tubular film positioned in the bag machine, the blade and heat sealing assembly is actuated. The assembly moves downwardly to cause the heating element to engage the film and the blade to penetrate the film. The assembly then moves upwardly to the retracted position permitting the film to be indexed to the next position, wherein the heat sealing and perforation cycle is repeated. A commercially available machine that operates in this manner is the Gloucester Universal Bag Machine, marketed by Gloucester Engineering Co., Inc., located in Gloucester, Mass.
The in-line bag machine, as described above, functions satisfactorily for many types of film such as low density polyethylene (LDPE) film. However, it has been found that machines of this type are not entirely satisfactory for linear low density polyethylene (LLDPE) film in that the blade must be carefully maintained to provide satisfactory penetration. When the blade becomes even slightly dulled, it tends to push the film downwardly into the slot underlying the film and aligned with the cutting blade, resulting in incomplete perforations or if the film is fully supported in the underlying slot, it tends to weld together the edges of the slit formed by the blade penetration. Moreover, the downward pushing action increases the film tension in the heat sealing region and adversely affects the quality of the seal attainable.